Auto body repair & detailing: What kind of spray foam?, rust holes, pontiac grand prix

Question

Hi Ernie,

I red your answer to Colm Burke about repairing a hole in the bumper, You suggested to use spray foam before body filler application. I have to fix some rust holes bottom of the door frame of a 1994 Pontiac Grand Prix. Holes actually are hidden under a plastic cover, which covers all length of the chassis between bumpers.  What kind of spray foam can be used for this application? I will apply polyester fibreglass on top of it.


Answer
I am guessing the panel you are referring to is called a rocker panel.I would not recommend  repairing to much of this panel with foam.This panel is a structural part of the car and if there is too much rust the integrity of the car is compromised if you were to get into a collision the safety of the car is not there.Excessive rust should be patched with steel welded in.Any kind of expandible foam will work for smaller rust holes but there are products available from auto paint supply stores that are more moisture resistant and designed for auto repair.